1. Field of the Invention
The present invention relates to a process for producing 2,6-dimethylnaphthalene. More particularly, it pertains to a catalyst for producing 2,6-dimethylnaphthalene which is useful as a starting raw material for 2,6-naphthalene dicarboxylic acid.
2,6-Napthalene dicarboxylic acid has an industrially important use as a starting raw material for high-performance polyester, polyethylene naphthalate fibers and films having excellent tensile strength and heat resistance.
2. Description of Related Art
2,6-Dimethylnaphthalene has heretofore been obtained by isolating it from a coal tar fraction or a fraction of heavy oil subjected to fluid catalytic cracking (FCC). However, the aforementioned isolation process affords the fraction in the form of a mixture containing almost all the types of methyl group-position isomers such as 1,2-, 1,3-, 1,4-, 1,5-, 1,6-, 1,7-, 1,8-, 2,3- and 2,7-dimethylnaphthalene in addition to the objective 2,6-dimethylnaphthalene. Thus, the process for isolating and purifying 2,6-dimethylnaphthalene from such fraction mixture suffers from the disadvantage of a number of steps and a high cost required in the production thereof, making itself unsuitable for inexpensive mass-production of 2,6-dimethylnaphthalene.
There have recently been proposed several processes for producing 2,6-dimethylnaphthalene from a variety of starting raw materials, but there has not yet been established an industrial production process capable of efficiently and selectively synthesizing 2,6-dimethylnaphthalene by the use of inexpensive starting raw material available in a large quantity.
Examples of the above-mentioned process include the process disclosed in Japanese Patent Application Laid-Open Nos. 172937/1985, 29536/1987 and 8344/1988 in which naphthalene or monomethylnaphthalene is methylated into dimethylnaphthalene and the process disclosed in Japanese Patent Application Laid-Open Nos. 45536/1985 and 14737/1988 in which naphthalene or monomethylnaphthalene is transmethylated into dimethylnaphthalene by using polymethylbenzene.
Nevertheless, the process disclosed in any of the aforesaid laid-open patent applications suffers from the defects that the conversion of naphthalene and monomethylnaphthalene to 2,6-dimethylnaphthalene is low and selective production of 2,6-dimethylnaphthalene is difficult. Consequently, the process necessitates complicated isolating purification steps and intricate isomerization steps, thus causing disadvantageous problems from the industrial point of view.
In addition, Japanese Patent Application Laid-Open Nos. 61647/1973 and 48647/1974 disclose a process for production of 2,6-dimethylnaphthalene by cyclization, dehydrogenation and isomerization of 5-(o-tolyl)pentene-2 to be used as a starting material. Moreover, Japanese Patent Publication Nos. 17983/1975, 17985/1975 and 22550/1975 disclose a process for producing dimethylnaphthalene by the cyclization dehydrogenation of 5-(o-tolyl)pentene-2.
In the above-mentioned processes, however, 5-(o-tolyl)pentene-2 to be used as a starting material is usually produced from o-xylene and 1,3-butadiene by the use of an alkali metal such as potassium or sodium as a catalyst, and many problems remain unsolved with, regard to the catalyst handling, especially with respect of safety. Likewise, as the resultant dimethylnaphthalene is obtained as the mixture of isomers such as 1,5-dimethylnaphthalene, 1,6-dimethylnaphthalene, 2,6-dimethylnaphthalene and the like, the production of 2,6-dimethylnaphthalene therefrom suffers a lot of disadvantages that the steps of isomerization, separation and purification are necessary.
On the other hand, attempts have been made to selectively produce 2,6-dimethylnaphthalene by cyclization and dehydrogenation of 2-methyl-1-(p-tolyl)butene or 2-methyl-1-(p-tolyl)butane as disclosed, for example, in Japanese Patent Publication No. 5292/1978 wherein a catalyst comprising rhenium oxide, an alkali metal oxide or alkaline earth metal oxide and alumina is used, and in Japanese Patent Publication No. 1701/1976 wherein a catalyst of chromia/alumina series containing an alkali metal oxide is employed.
However, the above-disclosed catalysts are unfavorable, since the use thereof results in a low yield and an insufficient purity of the objective 2,6-dimethylnaphthalene; besides the highly toxic chrominum compound contained therein will bring about environmental pollution problems.
It has been disclosed by the present inventors, in Japanese Patent Application Laid-Open Nos. 173834/1991 and 251545/1991, that in the production of 2,6-dimethylnaphthalene by cyclization dehydrogenation of 2-methyl-1-(p-tolyl)butene or 2-methyl-1-(p-tolyl)butane, 2,6-dimethylnaphthalene with high purity is obtained at a relatively high yield by the application of a catalyst comprising lead and alumina incorporated with a third component and a catalyst comprising indium and alumina incorporated with a third component. Nevertheless, the above-developed catalysts still involve some problems such that the catalyst containing a lead component is industrially unfavorable due to its toxicity and the catalyst containing an indium component sometimes evaporates away during its use owing to the high volatility of a monovalent indium compound formed during the reaction and the like. Under such circumstances, it has been sought for a long time to develop a catalyst capable of producing highly pure 2,6-dimethylnaphthalene at a high yield with a stabilized operation without causing any problem relating to sanitation.
In the case where 2-methyl-1-(p-tolyl)butene or 2-methyl-1-(p-tolyl)-butane as a raw material is reacted by the use of a catalyst consisting of aluminum oxide alone, dimethylnaphthalene is generally produced but the yield thereof is too low and unfavorable side reaction such as isomerization, decomposition and polymerization of a raw material are too excessive to make the catalyst industrially usable.
The application of a catalyst consisting of aluminum oxide incorporated with a palladium component to the above reaction improves the selectivity to cyclization dehydrogenation and increases the yield of dimethylnaphthalene, but remarkable side reactions such as isomerization and demethylation of the resultant dimethylnaphthalene takes place together with decomposition and polymerization of a raw material, thus unfavorably lowering the selectivity to the objective 2,6-dimethylnaphthalene.
On the other hand, the application of a catalyst consisting of an alkali metal component or an alkaline earth metal component each supported on aluminum oxide can suppress decomposition and polymerization of 2-methyl-1-(p-tolyl)butene or 2-methyl-1-(p-tolyl)butane as a raw material, but the catalyst scarcely exhibits cyclization dehydrogenation activity or even if it exhibits the activity, the yield of dimethylnaphthalene is too low to make the catalyst industrially usable.
Specifically, the use of a catalyst consisting only of aluminum oxide unfavorably causes a variety of side reactions due to acid points on the surface of the catalyst including isomerization, decomposition and polymerization of a raw material to take place in the reaction process.
In view of the above, intensive research and investigation were concentrated by the present inventors in order to solve and overcome the above-mentioned problems. As a result, it has been discovered by the present inventors that highly pure 2,6-dimethylnaphthalene is obtained at a high yield by the use of a catalyst comprising a palladium component, at least one compound selected from alkali metal compounds and alkaline earth metal compounds and aluminum oxide. The present invention has been accomplished on the basis of the above-mentioned finding and information.